I have been looking at a number of sources regarding the contact activation form of the coagulation cascade, and I have seen conflicting information. I am hoping somebody that knows this area well can answer this.

In human blood coagulation Factor XII is converted to XIIa in a calcium-independent process, and Factor XIa, in the presence of Ca++ activates Factor IX.

What I am confused about is whether the conversion of XI to XIa is calcium-dependent or calcium-independent. Most of the references I have consulted have not mentioned Ca dependence, but I found one chart that showed that XI to XIa was dependent. Was that a typo on the chart? The chart souce was a web site that is not particularly authoritative and I suspect it is a well intentioned (but incorrect) attempt to describe the process.

Also, if anybody can explain to me why heparin inhibits the conversion of XII and XI to the active forms, that would be helpful. I understand that heparin increases the binding affinity of ATP by more than 1,000 fold but I also have seen references that heparin increases TPA activity. Since TPA, by itself, can convert plaminogen to plasmin it is not clear to me how heparin can inhibit XIIa conversion while increasing TPA.

I am asking all this because I don't care about coagulation (at all) but I do care about plasminogen activation so I need to understand what does, and does not, get in the way of plasminogen to plasmin activation.